Many vehicles nowadays have some form of suspension system. A complete suspension system usually comprises a spring and a damper. The spring may be a steel helical spring and a damper may be disposed inside the spring. The total force in the spring leg is given by the sum of the force from the spring and the force from the damper. The spring force is determined by the position of the spring leg and the damping force is given by the speed of the spring leg. This principle is currently applied to most types of wheeled vehicles, such as passenger cars and heavy-duty vehicles.
For heavy-duty vehicles, gas-hydraulic (hydropneumatic) suspension is used in certain cases. A suspension system of hydropneumatic type often comprises a hydraulic cylinder, filled with hydraulic liquid, which is connected to an accumulator, which has two chambers separated by a piston. One of the chambers, on one side of the piston, contains hydraulic liquid, and the other chamber, on the other side, contains a pneumatic pressure medium, which constitutes the actual spring element. Damping valves are arranged along the flow path of the hydraulic liquid between the hydraulic cylinder and the accumulator, the function of said damping valves being to create a flow resistance when the hydraulic liquid is forced to flow through the damping valves. In this way, a damping action is exerted on the spring motion of the vehicle. In this case, the accumulator may be provided with a separating piston between the hydraulic fluid, which is essentially incompressible, and the pneumatic medium, which is compressible.
These prior art suspension units have resulted in increased comfort, better driving characteristics, less vibration, reduced effect upon the chassis and increased productivity, especially when the vehicle has been heavily loaded.
Another common solution in the field of vehicle suspension is the use of bellows cylinders. These have a simpler design and are often driven pneumatically, but they may also be driven by other fluids and mechanical springs. In the case heavily loaded heavy-duty vehicles, bellows cylinders are not appropriate, since they are not capable of cushioning the load in an optimal manner. This has the effect that the dynamic behavior of the vehicle is restricted, thus affecting the drivability of the vehicle.
The newer types of suspension systems have not been entirely beneficial. As a result of progress, the suspension systems have become increasingly complex with more electronic components being built into the solutions, with the result that the systems have become more expensive. Also the weight of the suspension systems has increased as a result of all the added functionality. Consequently, the environmental impact of the vehicle increases in the form of increased emissions and costs.
In order to satisfy future requirements for environmental compatibility while maintaining performance unchanged, there is a need for suspension systems which are more compact and lighter, but which offer the suspension properties needed to ensure improved comfort and better driving dynamics for heavy-duty vehicles.
Swedish Patent SE 444 541 discloses a hydropneumatic suspension system, which comprises an outer and an inner cylinder, which initially has two well-defined chambers pre-pressurized by means of different mediums (e.g, oil and gas, respectively, etc.). During a given displacement of the suspension system, a globe valve is opened, whereupon the pressure in the two chambers is equalized for the purpose of changing the suspension curve.
In view of the current state of suspension systems, there is a need for suspension systems which permit the setting of spring characteristics and road clearance and which are simple and robust and can be manufactured at a lower cost.